Calender bank size automatic control system



Sept. 20, 1966 J. T. FREEMAN ET L 3,274,308

CALENDER BANK SIZE AUTOMATIC CONTROL SYSTEM Filed Jan. 2, 1964 2 Sheets-Sheet l INVENTORS JOHN THOMAS FREEMAN CARL WILLIAM SANDERS BY v awfl AGENT Sept. 20, 1966 J. T. FREEMAN ET AL 3,274,308

CALENDER BANK SIZE AUTOMATIC CONTROL SYSTEM Filed Jan. 2, 1964 2 Sheets-Sheet 2 FIG. 2

INVENTORS JOHN 'THOMAS FREEMAN CARL WILLIAM SANDERS AGENT United States Patent Sanders, Newark, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Jan. 2, 1964, Ser. No. 335,220 7 Claims. (Cl. 264-40) This invention relates to a method for continuously controlling the size of a bank in the nip between two calender rolls and also to apparatus for carrying out the method.

In the formation of some viscous products by a calendering operation it is important that the final product be free from air bubbles. This is particularly true in the manufacture of sheet or webs of thermoplastic compositions such as photopolymer. The method of the invention will be illustrated and described in connection with the manufacture of photopolymerizable compositions and elements such as disclosed in assignees US. patents, Plambeck US. 2,760,863 and 2,791,504; Barney US. 2,893,868; Martin US. 2,902,365; Martin et al. US. 2,927,022; and Smith U.S. 3,012,952. It should be understood that such illustrations and descriptions are merely exemplary in that the invention may be adapted with little or no modification to other calendering operations where the quality of the final product is related to bank size.

In the manufacture of photopolymerizable compositions it has been found that there is a close relationship between the size of the calender bank and the incidence of bubbles. The feed involved in the calendering of photopolymer is in the form of small particles or chips. During the pass of the feed through the first nip in the calendering operation, many small pockets of air are trapped in the web created. These air pockets will be worked out of the web as they revolve in properly sized banks in the subsequent nips in the calender. If the banks involved are too small, they will tear and trap additional air which will be found in the final product as bubbles. Additionally it has been found that if the bank size is too large, layers known in the art as pancakes will form on the surface of the bank. The pancakes can not be used and must be removed, usually by hand. Therefore it is desirable to obtain and maintain the optimum bank size for the material being calendered.

The prior method of controlling bank size was by the direct visual observation or indirect observation via television by a trained operator. The operator upon observing the bank and mentally determining that the size was either too small or large would either increase or decrease the size of the initial nip to regulate the flow of polymer or would adjust the feed rate to obtain the desired bank size. This method has many drawbacks, and one is that it requires the uninterrupted attention of a skilled operator. In many calendering operations involving plastic materials the diameter of the part of the bank that can be observed is between A" and and, even with skilled operators, the changes in bank size that will effect the formation of bubbles are very slight and in many instances cannot be detected by visual observation. With this method, because of the human element involved, a change in operators will usually result in a change in what is considered the optimum bank size. Additionally, this method does not produce any permnaent record that can be used to determine optimum bank size under various conditions.

It is an object of this invention to provide an apparatus to automatically control calender bank size. A further object is to provide an apparatus for controlling bank size which is easily adaptable to existing calender rolling operations. Another object is to provide a method for accurately controlling calender bank size. Other objects will appear hereinafter.

These and other objects are accomplished by an apparatus which comprises means for sensing the bank size of the material in the nip between two calender rolls which produces a signal corresponding to the size of the bank, means for developing a control signal by comparing said bank size signal with a constant optimum bank size signal, and means for changing the distance between the calender rolls upon receipt of said control signal thereby varying the bank size.

The invention is best explained by means of the attached drawings which are a part of the specification, however, the invention is not intended to be limited to these drawings or the following preferred embodiments as designated in the disclosure except as prescribed in the claims.

FIG. 1 is a side elevation of a calendering operation;

FIG. 2 is a schematic diagram of the preferred apparatus for carrying out the method of the invention.

Referring now to FIG. 1, in an inclined Z, four roll calender the charge 11 is fed into the first nip 12 between calender rolls 1 and 2. The charge is transported to the calender by a conveyor 13. After the charge passes through the first nip it assumes the form of a film or web 14. This web then passes through two successive nips 15, 16 wherein it is further mixed and reduced to the prescribed gauge. Transfer from one roll to the next is generally accomplished by some combination of temperature, speed or surface-finish differential familiar to those skilled in the art. The web is then stripped by a smaller roller 17 and is then ready for further processing.

In the illustrated inclined calender the number 3 roll is fixed and rotates in the direction shown. The other three rolls are movable in the directions indicated, this allows the size of the three nips through which the web passes to be adjusted. Additionally the rolls may be skewed, i.e., the axis of one roll slightly rotated out of the plane of the other. This increases the nip opening at the ends of the rolls relative to that at the center and com- :pensates for the deflection of the rolls.

In the second (15) and third (16) nips the flow properties of the material in the charge and the size of the nip result in a buildup 18 of the material all along the length of the nip. This buildup is called the calender bank or cigar by those familiar with the art. The banks are either folding banks or rotating banks depending upon the flow properties of the material involved. In the manufacture of photopolymerizable elements the bank that is formed is a rotating bank and the bank usually extends most of the length of the nip. The size of the bank may be uniform along the whole length of the nip or it may be largest at the center and taper in size toward the nip ends.

In the manufacture of photopolymer it is the size of the revolving banks 18 that must be controlled to prevent the formation of bubbles and pancakes. In the illustrated embodiments FIG. 1 the bank size is being measured by bank size sensing devices 19 at the second (15) and third (16) nips. In some calendering operations it will be suificient to control the bank size at only one nip; i.e., the one closest to the initial nip of the calender, for the required quality control.

A suitable apparatus used to measure the calender bank size is disclosed in assignees copending US. patent application to Freeman, filed January 2, 1964, Serial No. 335,218. In its preferred embodiment the measuring device is basically \a roller-like probe that rides on the surface of the revolving bank near its center. The roller is mechanically linked to a soft iron core that moves within a differential transformer such that a change in bank size results in a proportional change in the voltage induced in the transformer.

Referring now to FIGURE 2 which illustrates the preferred embodiment of the invention; the illustration shows controlling the bank size in only one nip of the calender.

The output of the differential transformer of the sensing device 19 is an AC. signal and has a magnitude that varies as a function of the size of the bank. The output signal is transmitted to an A.C. to DC. silicon diode half wave rectifier 20 wherein it is converted into a DC. signal having a range of 50 mv., and if desired, to a DC. voltage recorder 21. A 0 to rnillivolt voltmeter with a strip recorder such as made by Honeywell may be used.

The DC. signal is transmitted to an electrical/pneumatic transducer 22. This transducer produces a proporrtional pneumatic output signal in the range of 3 to p.s.i. from the DC. input. A suitable transducer unit is a Moore Products Mod. 7711 E/ P Transducer a description of which can be found in Bulletin 7701 published by Moore Products, Philadelphia, Pennsylvania.

The pneumatic signal is sent to a proportional plus reset pneumatic controller 23. This controller first, in analog fashion, compares the signal representing the measured bank size or primary feedback with a reference signal. The reference signal is a manually set signal and it represents the desired or optimum bank size. Then if there is any difference between the two signals, the difference is amplified within the controller. Thus for a given deviation from set point, the controller output contains the deviation multiplied by a factor dependent upon the gain setting of the controller and the deviation times a factor dependent upon the reset setting of the control integrated with respect to time. A controller that can be used for this purpose is the Moore Products Nullmatic, Model 56M. A description of the controller and its operation may be found in the Instruction Book SD50-3 entitled Nullmatic Controllers published by Moore Products Co., copyrighted in 1958 and in the following US. Patents 2,312,201; 2,359,236; 2,518,244; and 2,520,468.

The controller output signal is transmitted to a roll positioner 24. This device senses the angular position or rotation of shaft 27 connected to the roll positioning motor 28, i.e., the motor that controls the nip size. The position then develops a spring force that is proportional to the sensed angular position. The spring force is compared within the roll positioner with the controller output signal and any difference results in a final control signal that is used to actuate motor 28. The use of the roll positioner is particularly important. The positioner gives a controlled variable whose magnitude changes as the time integral of the difference between the input and output feed rates of the particular nip for which the bank size is being measured (e.g. for a given input polymer feed rate if the particular calender clearance at the nip caused a lower removal rate, the buildup would continuously increase with time). If it were not for this control variable, the nip would have to be modulated in proportion to the deviation in the desired quantity of bank buildup or the system will either cycle excessively or perform in a substantially inferior manner. A suitable roll positioner is described in Product Specification P8 l-1 and P99-5 published by Bailey Meter Co., copyrighted in 1954.

The roll positioner 24 is connected to the shaft 27 by coupling means 31 such as a differential drive unit or an electric slip coupling. When the electric slip is utilized, the coupling is disengaged during initial start-up of the machine since the shaft 27 is rotated through several turns during this time and exceeds the range of travel of the mechanical linkage of the roll position 24. When the machine has been brought into approximate balance, the electric slip coupling is energized and the control system maintains the appropriate nip clearance. Since small angular movements of the shaft 27 are critical, it is necessary to match the desired range of shaft 27 rotation with the mechanical sensitivity of the roll positioner 24 to assure control performance is not significantly degraded by the backlash, friction, etc. within the roll positioner 24. A suitable electric coupling type FFK60 is manufactured by Simplatrol Products Corporation.

The control signal from the roll positioner is used to operate two pressure switches 25 and 26 that are in the electrical circuit that supplies the power to the roll positioning motor 28. Thus if the angular position of the shaft is correct for the bank size desired, the output of the positioner will not open-ate either of the pressure switches and the nip or roll positioning motor will not be activated. If the output indicates an error in the shaft position, depending on which error is involved one of the pressure switches will close and the nip motors will run either in or out to correct the error.

The nip motor operates a pair of lead screws (not shown) one at each end of the [calender and connected to each other by suitable gearing means 29. Thus the nip will be altered simultaneously at each end of the calender.

The operation of the entire control unit may be cycled with respect to the operation of the calendering unit, in order to prevent the roll positioning motor from continuously operating and possibly overheating. The power input to the roll positioning motor is transmitted through a manual set proportional timer 30 and the nip motors are only activated for a portion of the time the unit is in operation, e.g., the automatic control unit can be in operation for 20% of a 30 sec. basic cycle or a 60 sec. basic cycle.

The method of the invention has been illustrated by the use of a feeler type sensing device in conjunction with a differential transformer. But it is not intended to limit the method to such a system. Other means that continually and accurately sense the bank size and develop a signal proportional to the bank size may be used without departing from the spirit of the invention. In the application involving photopolymer the physical contact between the feeler and bank does not damage the final Web, but if such contact would damage the web other means could be used. The size of the bank could be sensed by an air gauge, ultraviolet radiation, nuclear radiation, sound absorption, or dielectric absorption and a corresponding signal developed.

Many modifications of the preferred embodiment will be obvious to those skilled in the art and within the scope of the present invention. Thus electronic means could be used instead of the pneumatic means, e.g., instead of a pneumatic controller an electrical summer circuit and an operational amplifier, an electrical or hydraulic controller could be used to develop the correcting signal. An electrical positioning unit could sense the angular position of the shaft and operate relays in the power unit input circuit and electrical relays could be used instead of pressure switches. Derivative action could be added to further improve control response in any of the embodiments of this invention. Another alternative having similar performance would be to use a nip clearance measurement for the input to the positioning mechanism 28 rather than angular shaft position.

This invention has many advantages over the visual system of bank size control with operator adjustment in that it greatly multiplies the change in bank size, making it instantly apparent when a small change in bank size has occured and has reliable apparatus for correcting the bank size. Another advantage of the invention is that it provides a necessary tool to determine and maintain the optimum bank size to close dimensions for any material being calendered.

We claim:

1. A calendar roll apparatus for continuously preparing viscous sheet material comprising (A) detector means for signalling the bank size of said material formed by the nip of said calender roll;

(B) means of creating a deviation signal representing the difference between said bank size signal and a preset signal for the optimum bank size; and

(C) control means for governing the nip size by moving said calendar roll a distance dependent upon a correction signal representing the difference between a signal corresponding to the actual nip size and said deviation signal.

2. An apparatus for continuously preparing viscous sheet material comprising (A) calender rolls;

(B) a motor for laterally separating said rolls to control the nip between the rolls;

(C) detector means for signalling the bank size of said material created by the nip between said rolls;

(D) means for creating a deviation signal representing the difference between said 'bank size signal and a pre-set signal for the optimum bank size; and

(E) control means for operating said motor by issuing a corrective signal to the motor which represents the difference between a signal corresponding to the nip size of said calender rolls and said deviation signal thereby correcting the nip size to maintain optimum bank conditions.

3. A method for preparing continuous sheets of photopolymeric material which comprises (A) feeding said material between calendar rolls capable of being adjusted to change the nip size;

(B) producing a signal representing the actual calender bank size in said nip;

(C) comparing the actual bank signal with a preferred bank size signal and forming a deviation signal for any difference in said bank signals; and

(D) adjusting said calender roll nip utilizing said deviation signal to give said preferred calender roll bank size.

4. A method for preparing continuous sheets of photopolymeric material which comprises (A) feeding said material between calender rolls capable of being adjusted to change the nip size;

(B) producing a signal representing the actual calender bank size in said nip;

((3) diiferentiat-ing said actual bank size signal with a preferred bank size signal to give a deviation signal representing the difference between said actual and preferred bank size signals;

(D) comparing said deviation signal with a signal representing the nip distance of said calender rolls and issuing a corrective signal; and

(E) adjusting said nip distance of said calender rolls with said corrective signal.

5. Apparatus as defined in claim 2 where said control means issues a correction signal for said motor which represents the difference between a signal corresponding to the angular position of the drive shaft of said motor and said deviation signal.

*6. An apparatus as defined in claim 5 having (F) timing means to allow said correction signal to operate said motor tor predetermined lengths of time.

7. An apparatus as defined in claim -5 where said (D) means for creating a deviation signal comprises a transducer means for converting said detector signal into a pneumatic signal and creating a deviation signal for the difference between said detector signal and a preset pneunatic signal for the optimum bank size.

References Cited by the Examiner UNITED STATES PATENTS 2,678,465 5/1954 Schnuok et al. 2,736,217 2/1956 Blain 33-182 2,809,519 10/ 1957 Kaestner 73-159 3,149,421 9/ 1964 OBrien.

References Cited by the Applicant UNITED STATES PATENTS 2,809,519 -10/ 1957 Kaestner.

2,858,614 11/1958 Schuenemann.

ROBERT F. WHITE, Primary Examiner.

S. A. HELLER, Assistant Examiner. 

1. A CALENDAR ROLL APPARATUS FOR CONTINUOUSLY PREBANK SIZE SIGNAL AND FORMING A DEVIATION SIGNAL FOR PARING VISCOUS SHEET MATERIAL COMPRISING (A) DETECTOR MEANS FOR SIGNALLING THE BANK SIZE OF SAID MATERIAL FORMED BY THE NIP OF SAID CALENDER ROLL; (B) MEANS OF CREATING A DEVIATION SIGNAL REPRESENTIING THE DIFFERENCE BETWEEN SAID BANK SIZE SIGNAL AND A PRESET SIGNAL FOR THE OPTIMUM BANK SIZE; AND (C) CONTROL MEANS FOR GOVERNING THE NIP SIZE MOVING SAID CALENDAR ROLL A DISTANCE DEPENDENT UPON A CORRECTION SIGNAL REPRESENTING THE DIFFERENCE BETWEEN A SIGNAL CORRESPONDING TO THE ACTUAL NIP SIZE AND SAID DEVIATION SIGNAL.
 3. A METHOD FOR PREPARING CONTINUOUS SHEETS OF PHOTOPOLYYMERIC MATERIAL WHICH COMPRISES (A) FEEDING SAID MATERIAL BETWEEN CALENDAR ROLLS CAPABLE OF BEING ADJUSTED TO CHANGE THE NNP SIZE; (B) PRODUCING A SIGNAL REPRESENTING THE ACTUAL CALENDER BANK SIZE IN SAID NIP; (C) COMPARING THE ACTUAL BANK SIGNAL WITH A PREFERRED ANY DIFFERENCE IN SAID BANK SIGNALS; AAND (D) ADJUSTNG SAID CALENDER ROLL NIP UTILIZING SAID DEVIATION SIGNAL TO GIVE SAID PREFERRED CALENDER ROLL BANK SIZE. 